J)KTEKMINAT10\ (JF PHOSPHORIC OXIDE. 361 



is evident that if the ratio of 50 niols of sodium hydroxide to i niol 

 of ph<_)sphoric oxide is correct, the factor for conversion of 

 cubic centimetres of N/io sodium hydroxide to grams of ])hos- 

 . phoric oxide is 0.000284. 



The effect of using the factor 0.00284 instead of the deter- 

 mined factors for each of the phosphates analysed is shown in 

 the following table : — 



Phosphoric Oxide. 

 Substance. Theory. Gravimetric VoUiinetric 



Determination. Detennination, 



Percent.- Percent. Percent. 



I.— CaHP()4 . 2H2O ... 41.27 ... 41-53 ■■■ 41 -.32 



2.— CaPIPO* . 2HaO 41.27 ... 41.14 ... 41.32 



3. — CaHPOi . 2HaO 41.27 ... 41.40 ... 41.21 



4. — KIIjPOj o-i!^ ••• 52. 4J. -.. 52.01 



5.— CaHP04 . 2HjO ... 41.27 ... 41.24 ... 41 -?4 



6.— CaliPO^ . 2H2O 41.27 ... 41.34 .• 41. '8 



7. — Mixed I'"ertiHser — ... 4.23 . . . 4.24 



8. — Mixed Fertiliser — ... 3.07 ... 3.03 



9. — Superphosphate — ... 17.02 ... 16.90 



None of the results are outside the limits of experimental error. 

 The agrement between duplicates in the voliunetric is close, a 

 difference of more than o. i c.c. in a titration of 50 c.c. being rare. 

 If the amount of substance taken for analysis is o.i gram, as is 

 the usual practice in fertiliser analysis by this method, an error 

 of 0.3 c.c. in the titration affects the percentage result by less 

 than 0.1. 



Experiments were carried out in order to ascertain the effect 

 of citric acid and ammonium citrate in the phosphate solution on 

 the quantity of sodium hydroxide used. These experiments show 

 that when precipitation takfes place in the presence of these 

 substances the amount of sodium hydroxide used in the titration 

 is slightly more than that required when citric acid and ammonium 

 citrate are absent. The excess of sodium hydroxide used is, 

 liowever, so small that the results are not materially affected. In 

 the following table some of these results are given : — 



Phosphoric Oxide. 

 Phosphate used. Theory. Gravimetric Volumetric 



Determination. Determination, 

 gr. gr. gr. 



CaHP04 . 2H2O dissolved in 



Ammonium Citrate 0.00825 ... 0.00831 ... 0.00832 



CaHPO* . 2H3O dissolved in 



2 per cent. Citric Acid ... 0.02063 — 0.02076 



Basic Slag, 2 per cent. Citric 



Acid Extract — ... 0.01584 ... 0.01592 



CaHPO^ . 2H..O dissolved in 



Ammonium Citrate 0.00825 ... 0.00828 ... 000.832 



Pemberton.* Kilgoref, and others have drawn attention to 

 the necessity for performing the solution of the ammonium phos- 



* Loc. cit. 



t U.S. Division of Chemistry Bull., 43 (r8g4). 



